One method for controlling a three-phase motor to revolve at a desired angular velocity is the vector control. The vector control is a method in which a three-phase motor (such as, induction motors, synchronous motors and permanent magnet motors) is handled in the same way as a dc machine by representing the electrical status (such as, current and voltage) of the secondary winding with two separate components: the direct axis component (or d-axis component) and the quadrature axis component (or q-axis component), and controlling the individual components.
FIG. 1 is a block diagram showing a three-phase motor system including a three-phase motor control apparatus which performs a vector control on a three-phase motor on the basis of a d-q axis transformation. The three phase system of FIG. 1 includes a three-phase motor 101, an angle detector 102, a driver circuit 103 and a three-phase motor control apparatus 104. The three-phase motor control apparatus 104 includes a desired rotation angle generator 105, a feedback controller 106 and a two-phase to three-phase transformation section 108.
The operation of the three-phase motor system of FIG. 1 is schematically as follows: The rotation angle of the rotor of the three-phase motor 101 is detected by the angle detector 102. The rotation angle detected by the angle detector 102 is referred to as detected rotation angle θ, hereinafter. A desired value of the rotation angle of the rotor (desired rotation angle θ*) is generated by the desired rotation angle generator 105. The desired rotation angle θ* is generated so that the three phase motor 101, which is attached with a load, revolves at a desired angular velocity. The feedback controller 106 generates a d-axis command Vd* and a q-axis command Vq* by a feedback control in response to the desired rotation angle θ* and the detected rotation angle θ. Three-phase commands Vu*, Vv* and Vw* are generated by performing a two-phase to three-phase transformation on the d-axis command Vd* and the q-axis command Vq*, by the two-phase to three-phase transformation section 108. The driver circuit 103 is responsive to the three-phase commands Vu*, Vv* and Vw* for feeding three-phase power through three phase power lines 103a, to thereby drive three-phase motor 101. The three-phase motor system of FIG. 1, in which the feedback controller 106 performs a feedback control, such as a current control and a speed control, achieves the desired angular velocity.
Three-phase motor systems as shown in FIG. 1 are disclosed in various patent literatures. For example, Japanese Patent Gazette No. 3,244,144B (patent literature 1) discloses a configuration in which a PI control and a neural network are used for the feedback control. Japanese Patent Application Publication No. H09-215399 A (patent literature 2) discloses a configuration in which a speed control is performed as the feedback control to calculate the q-axis command. Furthermore, Japanese Patent Application Publication No. 2006-149176 A (patent literature 3) discloses that the feedback controller includes a current controller or speed controller as well as a repetition controller.
In addition, Japanese Patent Application Publication No. H07-170799 A (patent literature 4) discloses a technique for improving the preciseness of the control and avoiding torque ripple by correcting the error (the offset value or the non-linear error) in a current detector provided within a feedback control system. Furthermore, Japanese Patent Application Publication No. H08-322299 A (patent literature 5) discloses a technique for generating function values of trigonometric functions used in the feedback control with a reduced memory capacity. In addition, Japanese Translation of PCT Application No. WO2005/067137 (patent literature 6) discloses a technique for precisely calculating the rotation angle by correcting the detected value of the rotation angle (or the position).
One issue of the three-phase motor system shown in FIG. 1 is how to deal with the torque disturbance which is synchronous with the rotation of the rotor of the three-phase motor 101 (hereinafter, referred to as “rotation synchronous torque disturbance”). The torque may vary in synchronization with the rotation when there is an imbalance in the distribution of the moment of inertia with respect to the rotation center of the rotor of the three-phase motor 101, due to the offset with respect to the rotation center of the rotor shaft of the three-phase rotor and the like. In other words, the rotation of the rotor of the three-phase motor 101 is influenced by the rotation synchronous torque disturbance. Due to the rotation synchronous torque disturbance, the vector control results in that the rotation angle of the three-phase motor 101 is not identical to the desired rotation angle θ*. In other words, the angular velocity of the rotor of the three-phase motor 101 does not coincide with the desired angular velocity.
One approach for regulating the rotation angle of the three-phase motor 101 to the desired rotation angle θ* may be to set the control frequency range of the feedback control to be in a higher range, that is, to increase the operation frequency of the angle detector 102 and the three-phase control unit 104. This approach is however insufficient, since the actually achievable control frequency range is limited in view of the limitation of the sampling frequency of the angle detector 102 and the necessity of the prevention of the oscillation of the feedback system. When the rotation synchronous torque disturbance is increased whereas the control frequency range of the feedback control system is limited, the difference between the desired rotation angle and the detected rotation angle may become larger than an allowed maximum value.